This invention relates generally to frequency measuring apparatus and more particularly to instantaneous frequency measurement (IFM) devices which are adapted for use in measuring the frequency of radio frequency signals.
As is known in the art, instantaneous frequency measurement (IFM) devices are used in a variety of applications as where it is desired to measure the frequency of individual pulsed or continuous wave (CW) radio frequency input signals. These devices generally include a wide bandwidth radio frequency limiter circuit fed by the radio frequency input signals where the IFM device is to operate over a dynamic range of 50 dB or greater. The limited radio frequency signals are then fed to at least one delay line discriminator to provide an unambiguous measurement of the frequency of the input signals. Each delay line discriminator generally includes a power divider for separating the limited radio frequency signals into two quadrature channels, each channel having two electrical paths, the signal in one such path being delayed in phase with respect to the signal in the other path, such phase delay being related to the frequency of the input signals. The outputs of the two quadrature channels are combined and detected to form two baseband frequency output signals which have amplitudes which are proportional to the sine and cosine of the phase difference between the signals in the pairs of electrical paths and hence related to the frequency of the input signals.
While such an IFM device is adequate for many applications, such devices are relatively expensive because of the large number of microwave devices included for power splitting, power combining and detection. Also, poor sensitivity results because of the large number of power splits prior to detection. For example, in an IFM device of the type discussed herein where say three delay line discriminators are used, each having a different ambiguity-free frequency range over the wide band of frequencies, only one-twelfth of the input power (less insertion losses) would be delivered to each one of twelve detectors. In addition, each detector is generally followed by its own baseband amplifier, thereby introducing additional baseband noise. Also, predetection noise due to the delay line included in the delay line discriminator and the quadrature channel signal combining is uncorrelated, each detector thereby increasing the baseband noise level. Further, because the microwave limiter fed by the input signal generally limits signals having moderately high signal levels, it is sometimes necessary to amplify the input signals to such high levels and then deliberately attenuate the limiter output to return to the range of the detectors, thereby increasing the cost of the IFM device. It is also noted that such an IFM device has numerous error sources, such as independent amplitude and phase error (frequency dependent) introduced by each power divider or power combiner and imperfections in the balance and matching of the detection.